Quantitative In-Line Analysis in Supercritical CO 2 Using Fibre-Optic NIR Spectroscopy and Multivariate Calibration: A Potential Method for Monitoring Continuous Flow Processes

Abstract

A set-up for fibre-optical in-line near infrared spectroscopic monitoring of hydrocarbons in supercritical CO₂ has been developed, which can be operated under flow-through conditions. It consists of a high-pressure optical cell, which is adapted to an FT-NIR spectrometer via a fibre-optic interface. NIR spectra of squalane in supercritical CO₂ have been measured for varying concentration, temperature and pressure conditions. A partial least squares (PLS) regression model was developed with a calibration set of 36 samples spanning the 0–32 × 10⁻³ g cm⁻³ squalane concentration range and with CO₂ densities between 0.62–0.86 g cm⁻³. Based on the corresponding absorbance data in the 4900–4200 cm⁻¹ spectral range, squalane and CO₂ were modelled with standard deviations of 0.24–10⁻³ and 3.8 × 10⁻³ g cm⁻³, respectively. Comparable standard deviations were obtained by validating the model with independent test samples. The developed calibration models allow for the in-line quantification of both constituents under varying temperature and pressure conditions from a single spectral measurement. The results obtained encourage the use of NIR spectroscopy/PLS calibration for fast in-line analysis of continuous flow extraction processes for the deoiling of metal parts based on supercritical CO₂.